Circuit boards are used to provide supported electrical connections for various components which are combined to form a system. Modern circuit boards have evolved from simple structures, such as described in U.S. Pat. No. 2,706,697 to Eisler, in which flat conducting lines are formed on one side of a sheet of insulating support material. As systems became more complex and increasing numbers of connecting lines became necessary, circuits were formed on both sides of insulating support sheets and then such circuit-bearing sheets were combined in layers with bonding or adhesive interlayers between them to form multilayer constructions of great complexity. Such multilayer boards and methods of construction are described in Chapters 20, 21, 22, and 23 of "Printed Circuits Handbook" edited by Clyde F. Coombs, and published by McGraw-Hill (second edition, 1979).
The circuit-bearing support sheet members used in making circuit boards have generally been made of thermosetting epoxy-glass or polyimide-glass, but these materials are high dielectric constant materials. While they are satisfactory for many applications, for applications in which high frequencies are used, or in which signals with very rapid rise times must be transmitted, performance would be improved by the use of insulating materials with lower dielectric constants. Reduction in dielectric constant has been achieved by using materials of lower dielectric constant. The epoxy-glass or polyimide-glass support sheets carrying printed circuits have been replaced with lower dielectric constant polytetrafluoroethylene-glass (PTFE-glass) in Japanese Kokai No. 60-257593.
To bind two or more copper-bearing support members, a variety of adhesives has been used. Such PTFE-glass circuit-bearing layers have been combined using lower melting thermoplastic layers as adhesives. Japanese Kokai No. 60-257593 uses and combines PTFE-glass circuit-bearing layers by introducing alternating layers of a perfluorinated melt-processible resin as an adhesive. Heat is applied to soften the thermoplastic portion, and pressure is used to exclude air and force the thermoplastic to conform to the circuitry. This method, however, causes movement of the circuits from their initial positions and consequent loss of essential spatial registration between circuit layers. Such movement of circuits is mentioned, for example, in U.S. Pat. No. 3,215,574 to Korb. To reduce such movement, which will sometimes herein be referred to as "lack of registration" of circuit bearing members, the Japanese Kokai apparently uses a glass cloth inserted into the adhesive perfluorinated melt-processible resin layer to reduce "swimming" or movement of the parts due to the plastic (molten) condition of the resin while under heat and pressure. Unfortunately, use of the glass cloth raises the dielectric constant of the adhesive layer. It would be beneficial to discover a construction and a method for preparing the multi-circuit board construction while maintaining registration of the circuit board members but yet which dispenses with the need for the glass cloth in the adhesive layer.
Furthermore, applicant conjectures that while heat and pressure are applied to bond the members of the multi-circuit board construction, the PTFE-glass support for the copper circuits stretches and deforms to some extent which would contribute to the ensuing lack of registration of circuits in the resulting board. It would be beneficial to discover a construction and a method for preparing it in which such stretching and deforming of the support does not occur.